This invention relates to digital spread spectrum communication in a variable propagation environment where there is radio frequency (RF) interference, such as in mobile communications in the high UHF bands. More particularly, the invention relates to a frequency hopping spread spectrum mobile receiver in the 900 MHz industrial band.
According to FCC regulations, the 900 MHz band is available under certain circumstances for industrial communications without special licensing. One mode of communications is spread spectrum, wherein a transmitted signal is spread over a frequency band that is much wider than the minimum bandwidth required to transmit particular information. This makes spread spectrum signals difficult to intercept and therefore suitable for private radio communication. There are two main spread spectrum techniques, direct sequence spectrum spreading and frequency hopping. Secure communication is provided by "tagging" the signal with a direct sequence or frequency hopping pattern that only the user's receiver recognizes.
In frequency hopping spread spectrum communication, the transmitter remains at a selected frequency for a few milliseconds dwell time, transmits useful information during a fraction of the dwell time, and then changes frequency to other frequencies in a predetermined pseudo-random pattern. The transmitter can be tracked by receivers having knowledge of the pseudo-random hopping pattern.
Spread spectrum communication is used in mobile communication systems which are often subject to poor RF environments. The poor RF environment arises because the receiver is located in a moving vehicle, subject to high ambient RF background noise, and blockage or interference from natural or man-made structures. Sporadic interferences may cause temporary loss of signal that may in turn cause the receiver to switch out of its signal tracking mode. The receiver is then required to restart the signal search and synchronization process. Moreover, existing synchronization methods typically employ an independent synchronization source such as a broad-band spectrum analyzer which increase the total signal acquisition time, as well as the cost and complexity of the receiver.
There is, therefore, a need for a low cost receiver capable of quick acquisition and reliable tracking of a frequency hopping digital signal transmitted in a poor RF environment.